Why and How do We Breathe?

When humans breathe in, air flows in via the mouth or nose. The air then follows the windpipe, which splits first into two bronchi: one for each lung. The bronchi then split into smaller and smaller tubes that have tiny air sacs at their end called alveoli (see Figure 1). We have millions of alveoli in our lungs! These sacs have thin walls—so thin that oxygen and carbon dioxide can pass through them and enter or leave our blood. The blood transports oxygen to almost every cell of the body. The blood picks up carbon dioxide released from the cells and gives it a ride back to the lungs. Carbon dioxide is released when we breathe stale air out.

Figure 1. Illustration of the parts of the human bronchi and lungs.

Many living organisms need oxygen to create energy within their cells. Oxygen is not essential for all living organisms, but humans adopted an oxygen-based metabolism because it is so efficient. When the cells use oxygen to create energy, they make carbon dioxide, a byproduct that needs to be disposed of. Humans get oxygen by filling their lungs with fresh air. The air we breathe in typically consists of about 78% nitrogen gas, about 21% oxygen gas, and less than 1% argon gas and traces of several other gasses. We remove carbon dioxide from the body by breathing out stale air. Because the human lungs extract about 15% of the oxygen we inhale, the air we exhale still contains about 17% oxygen. This lesson explores how air flows in and out of our lungs, compares lung breathing with other breathing mechanisms, and discusses why humans might have developed lung-breathing.

Relaxed breathing is a reflex; we do not have to think to breathe. During this unforced inhalation, our diaphragm—the dome-shaped muscle between the chest and the abdominal cavity—flattens. This expands the chest cavity and, as a result, air is drawn in (Figure 2). During exhalation, the diaphragm relaxes, the lungs naturally recoil, and air is gently pushed out, as shown in Figure 3.

Figure 2. Illustration showing how a flattened diaphragm initiates inhalation.

Figure 3. Illustration showing how the relaxation of the diaphragm initiates exhalation.

This dynamic works because of air pressure, which is a measure of how hard air presses against objects. Air pressure increases when you decrease the amount of space the air has, and decreases when you give air more space. Because air will move from areas of high pressure to areas where the pressure is lower—unless something blocks the movement—air rushes in or out of the lungs when we increase or decrease the size of the chest cavity. When the chest cavity expands there is more space for the lungs. In this condition the lungs can expand, making it a low-pressure area, and air rushes in to balance out the difference in pressure. Then to breathe out the chest cavity and lungs shrink. This increases the air pressure in your lungs, and the air rushes back out.

We can also breathe more forcefully. When we exercise, sing loudly, or otherwise need or want more air or oxygen we can exert force to breathe more deeply. We use various muscles to increase chest volume more dramatically. In the same way as in relaxed breathing, the expansion of the chest cavity draws air in so the lungs fill up. The relaxation of the chest cavity pushes air out. Muscles can also force the chest cavity to contract even further, pushing even more air out. Because the expansions and contractions are larger in this case, a bigger volume of air flows in and out of our lungs, and our body gets a larger supply of oxygen and we have more air to create sound.

Not all animals have developed lungs to breathe. Insects, centipedes, and arachnids use tracheal breathing. They have up to ten small breathing holes called spiracles. The spiracles allow air to enter smaller branches called tracheae, which allow for the oxygen and carbon dioxide exchange within the cells. Note that no blood or other transport mechanism is used to distribute oxygen throughout the body in tracheal breathers. Only small animals use tracheal breathing. Some animals with a thin and moist skin use skin breathing. Their skin is permeable enough to absorb oxygen and release carbon dioxide. These animals have thin blood vessels that transport the oxygen throughout the body. Sponges, corals, jellyfish, and worms breathe this way. Fish and crabs, on the other hand, use gill breathing. Gills allow the animal to absorb oxygen dissolved in water, and to deposit carbon dioxide into the water. Gills work for most aquatic animals, but are insufficient to sustain large aquatic animals like whales. Because 1 L of air contains a lot more oxygen than 1 L of water, large aquatic animals that need a lot of oxygen to sustain their cells developed lung breathing, just like humans did.

Whales do not breathe through their mouths, but developed blowholes. These holes are on the top of their heads, making it easier for them to breathe. They also developed distinct openings for eating and breathing. This allows them to eat underwater without getting fluid in their lungs when swallowing. Whales need to sustain a huge body, so they need a very efficient way to use the oxygen they inhale. Some researchers state that whales can use up to 90% of the oxygen they inhale! For humans, a 15% efficiency is enough. The human respiratory system still allows an exchange of large amounts of oxygen and carbon dioxide in a short time span, and the circulatory system allows distribution of this oxygen throughout the body. This helps us provide oxygen to nourish the 30 to 40 trillion cells making up the human body.

There is not always a clear-cut distinction between the types of breathers. Some animals use more than one type of breathing. Frogs, for example, use skin breathing and lung breathing. Other animals change their breathing type during their lives. When tadpoles morph into frogs, their breathing mechanism changes from gill breathing to lung and skin breathing.

In this lesson, students will explore the human respiratory system, and compare it to tracheal breathing, skin breathing, and gill breathing. The comparison will help them understand why humans developed lungs.